Track pre-commit hooks as Poetry dependencies - #397

This PR changes the way pre-commit is integrated with projects. Python-language hooks are no longer managed by pre-commit. Instead, they are tracked as development dependencies in Poetry, and installed into the Nox session alongside pre-commit itself. As development dependencies, they are also present in the Poetry environment.

In detail, this includes the following changes:

• Python-language hooks are converted to repository-local¹ system hooks².
• Hook dependencies are added to Poetry.
• Hook dependencies are installed into the Nox session for pre-commit.

¹ Repository-local hooks are not backed by an external git repository.
² System hooks are not installed into a pre-commit environment.

This approach has some advantages:

• All project dependencies are managed by Poetry.
• Hooks receive automatic upgrades from Dependabot.
• Nox can serve as a single entry point for all checks.
• Additional hook dependencies can be upgraded by a dependency manager. An example for this are Flake8 extensions. By contrast, pre-commit autoupdate does not include additional dependencies.
• Dependencies of dependencies (subdependencies) can be locked automatically, making checks more repeatable and deterministic.
• Linters and formatters are available in the Poetry environment, which is useful for editor integration.

There are also some drawbacks to this technique:

• This is not the officially supported way to integrate pre-commit hooks.
• The hook scripts installed by pre-commit do not activate the virtual environment in which pre-commit and the hooks are installed. To work around this limitation, the Nox session patches hook scripts on installation.
• Adding a hook is more work, including updating pyproject.toml and noxfile.py, and adding the hook definition to pre-commit-config.yaml.

See #316